3) Command line options
Driver parameters can be also passed in command line by using:

stmmaceth=dma_rxsize:128,dma_txsize:512

4) Driver information and notes

4.1) Transmit process
The xmit method is invoked when the kernel needs to transmit a packet; it sets
the descriptors in the ring and informs the DMA engine that there is a packet
ready to be transmitted.
Once the controller has finished transmitting the packet, an interrupt is
triggered; So the driver will be able to release the socket buffers.
By default, the driver sets the NETIF_F_SG bit in the features field of the
net_device structure enabling the scatter/gather feature.

4.2) Receive process
When one or more packets are received, an interrupt happens. The interrupts
are not queued so the driver has to scan all the descriptors in the ring during
the receive process.
This is based on NAPI so the interrupt handler signals only if there is work
to be done, and it exits.
Then the poll method will be scheduled at some future point.
The incoming packets are stored, by the DMA, in a list of pre-allocated socket
buffers in order to avoid the memcpy (Zero-copy).

4.3) Interrupt Mitigation
The driver is able to mitigate the number of its DMA interrupts
using NAPI for the reception on chips older than the 3.50.
New chips have an HW RX-Watchdog used for this mitigation.

On Tx-side, the mitigation schema is based on a SW timer that calls the
tx function (stmmac_tx) to reclaim the resource after transmitting the
frames.
Also there is another parameter (like a threshold) used to program
the descriptors avoiding to set the interrupt on completion bit in
when the frame is sent (xmit).

Mitigation parameters can be tuned by ethtool.

4.4) WOL
Wake up on Lan feature through Magic and Unicast frames are supported for the
GMAC core.

4.5) DMA descriptors
Driver handles both normal and enhanced descriptors. The latter has been only
tested on DWC Ether MAC 10/100/1000 Universal version 3.41a and later.

STMMAC supports DMA descriptor to operate both in dual buffer (RING)
and linked-list(CHAINED) mode. In RING each descriptor points to two
data buffer pointers whereas in CHAINED mode they point to only one data
buffer pointer. RING mode is the default.

In CHAINED mode each descriptor will have pointer to next descriptor in
the list, hence creating the explicit chaining in the descriptor itself,
whereas such explicit chaining is not possible in RING mode.

4.6) Ethtool support
Ethtool is supported. Driver statistics and internal errors can be taken using:
ethtool -S ethX command. It is possible to dump registers etc.

4.7) Jumbo and Segmentation Offloading
Jumbo frames are supported and tested for the GMAC.
The GSO has been also added but it's performed in software.
LRO is not supported.

4.8) Physical
The driver is compatible with PAL to work with PHY and GPHY devices.

4.9) Platform information
Several driver's information can be passed through the platform
These are included in the include/linux/stmmac.h header file
and detailed below as well:

struct plat_stmmacenet_data {

char *phy_bus_name;

int bus_id;

int phy_addr;

int interface;

struct stmmac_mdio_bus_data *mdio_bus_data;

struct stmmac_dma_cfg *dma_cfg;

int clk_csr;

int has_gmac;

int enh_desc;

int tx_coe;

int rx_coe;

int bugged_jumbo;

int pmt;

int force_sf_dma_mode;

int riwt_off;

void (*fix_mac_speed)(void *priv, unsigned int speed);

void (*bus_setup)(void __iomem *ioaddr);

int (*init)(struct platform_device *pdev);

void (*exit)(struct platform_device *pdev);

void *custom_cfg;

void *custom_data;

void *bsp_priv;
};

Where:
o phy_bus_name: phy bus name to attach to the stmmac.
o bus_id: bus identifier.
o phy_addr: the physical address can be passed from the platform.

If it is set to -1 the driver will automatically

detect it at run-time by probing all the 32 addresses.
o interface: PHY device's interface.
o mdio_bus_data: specific platform fields for the MDIO bus.
o dma_cfg: internal DMA parameters
o pbl: the Programmable Burst Length is maximum number of beats to
be transferred in one DMA transaction.
GMAC also enables the 4xPBL by default.
o fixed_burst/mixed_burst/burst_len
o clk_csr: fixed CSR Clock range selection.
o has_gmac: uses the GMAC core.
o enh_desc: if sets the MAC will use the enhanced descriptor structure.
o tx_coe: core is able to perform the tx csum in HW.
o rx_coe: the supports three check sum offloading engine types:

type_1, type_2 (full csum) and no RX coe.
o bugged_jumbo: some HWs are not able to perform the csum in HW for

over-sized frames due to limited buffer sizes.

Setting this flag the csum will be done in SW on

JUMBO frames.
o pmt: core has the embedded power module (optional).
o force_sf_dma_mode: force DMA to use the Store and Forward mode

instead of the Threshold.
o riwt_off: force to disable the RX watchdog feature and switch to NAPI mode.
o fix_mac_speed: this callback is used for modifying some syscfg registers

registers.
o custom_cfg/custom_data: this is a custom configuration that can be passed

while initializing the resources.
o bsp_priv: another private poiter.

For MDIO bus The we have:

struct stmmac_mdio_bus_data {

int (*phy_reset)(void *priv);

unsigned int phy_mask;

int *irqs;

int probed_phy_irq;
};

Where:
o phy_reset: hook to reset the phy device attached to the bus.
o phy_mask: phy mask passed when register the MDIO bus within the driver.
o irqs: list of IRQs, one per PHY.
o probed_phy_irq: if irqs is NULL, use this for probed PHY.

For DMA engine we have the following internal fields that should be
tuned according to the HW capabilities.

struct stmmac_dma_cfg {

int pbl;

int fixed_burst;

int burst_len_supported;
};

Where:
o pbl: Programmable Burst Length
o fixed_burst: program the DMA to use the fixed burst mode
o burst_len: this is the value we put in the register

supported values are provided as macros in

linux/stmmac.h header file.

---

Below an example how the structures above are using on ST platforms.

static struct plat_stmmacenet_data stxYYY_ethernet_platform_data = {

.has_gmac = 0,

.enh_desc = 0,

.fix_mac_speed = stxYYY_ethernet_fix_mac_speed,

|

|-> to write an internal syscfg

| on this platform when the

| link speed changes from 10 to

| 100 and viceversa

.init = &stmmac_claim_resource,

|

|-> On ST SoC this calls own "PAD"

| manager framework to claim

| all the resources necessary

| (GPIO ...). The .custom_cfg field

| is used to pass a custom config.
};

Below the usage of the stmmac_mdio_bus_data: on this SoC, in fact,
there are two MAC cores: one MAC is for MDIO Bus/PHY emulation
with fixed_link support.

static struct stmmac_mdio_bus_data stmmac1_mdio_bus = {

.phy_reset = phy_reset;

|

|-> function to provide the phy_reset on this board

.phy_mask = 0,
};

static struct fixed_phy_status stmmac0_fixed_phy_status = {

.link = 1,

.speed = 100,

.duplex = 1,
};

During the board's device_init we can configure the first
MAC for fixed_link by calling:
fixed_phy_add(PHY_POLL, 1, &stmmac0_fixed_phy_status));)
and the second one, with a real PHY device attached to the bus,
by using the stmmac_mdio_bus_data structure (to provide the id, the
reset procedure etc).

These can be read in several ways depending on the
type of the information actually needed.

For example a user can be use the ethtool support
to get statistics: e.g. using: ethtool -S ethX
(that shows the Management counters (MMC) if supported)
or sees the MAC/DMA registers: e.g. using: ethtool -d ethX

Compiling the Kernel with CONFIG_DEBUG_FS and enabling the
STMMAC_DEBUG_FS option the driver will export the following
debugfs entries:

/sys/kernel/debug/stmmaceth/descriptors_status
To show the DMA TX/RX descriptor rings

Developer can also use the "debug" module parameter to get
further debug information.

In the end, there are other macros (that cannot be enabled
via menuconfig) to turn-on the RX/TX DMA debugging,
specific MAC core debug printk etc. Others to enable the
debug in the TX and RX processes.
All these are only useful during the developing stage
and should never enabled inside the code for general usage.
In fact, these can generate an huge amount of debug messages.

6) Energy Efficient Ethernet

Energy Efficient Ethernet(EEE) enables IEEE 802.3 MAC sublayer along
with a family of Physical layer to operate in the Low power Idle(LPI)
mode. The EEE mode supports the IEEE 802.3 MAC operation at 100Mbps,
1000Mbps & 10Gbps.

The LPI mode allows power saving by switching off parts of the
communication device functionality when there is no data to be
transmitted & received. The system on both the side of the link can
disable some functionalities & save power during the period of low-link
utilization. The MAC controls whether the system should enter or exit
the LPI mode & communicate this to PHY.

As soon as the interface is opened, the driver verifies if the EEE can
be supported. This is done by looking at both the DMA HW capability
register and the PHY devices MCD registers.
To enter in Tx LPI mode the driver needs to have a software timer
that enable and disable the LPI mode when there is nothing to be
transmitted.

7) Extended descriptors
The extended descriptors give us information about the receive Ethernet payload
when it is carrying PTP packets or TCP/UDP/ICMP over IP.
These are not available on GMAC Synopsys chips older than the 3.50.
At probe time the driver will decide if these can be actually used.
This support also is mandatory for PTPv2 because the extra descriptors 6 and 7
are used for saving the hardware timestamps.

8) Precision Time Protocol (PTP)
The driver supports the IEEE 1588-2002, Precision Time Protocol (PTP),
which enables precise synchronization of clocks in measurement and
control systems implemented with technologies such as network
communication.

In addition to the basic timestamp features mentioned in IEEE 1588-2002
Timestamps, new GMAC cores support the advanced timestamp features.
IEEE 1588-2008 that can be enabled when configure the Kernel.

9) SGMII/RGMII supports
New GMAC devices provide own way to manage RGMII/SGMII.
This information is available at run-time by looking at the
HW capability register. This means that the stmmac can manage
auto-negotiation and link status w/o using the PHYLIB stuff
In fact, the HW provides a subset of extended registers to
restart the ANE, verify Full/Half duplex mode and Speed.
Also thanks to these registers it is possible to look at the
Auto-negotiated Link Parter Ability.